1. Field of the Invention
The present invention relates to a hydrostatic continuously variable transmission having a hydraulic pump and a hydraulic motor which are associated with each other via a hydraulic closed circuit.
2. Description of the Prior Art
A hydrostatic continuously variable transmission of this kind has already been known and it has been disclosed in Japanese utility model publication Kokoku No. 28902/86, for example.
Conventionally known hydrostatic continuously variable transmissions of this type use an input shaft arranged at the central portion thereof and a hydraulic pump driven by this input shaft is related at its outer periphery to a hydraulic motor concentrically disposed around the pump. This arrangement inevitably causes the motor cylinder of the hydraulic motor to have a large diameter and therefore it is not possible to provide on the outer periphery of the motor cylinder an output gear of such a relatively small diameter as would be suitable for driving a loading member at a reduced speed. Since in this arrangement an output shaft is continuously extended from the motor cylinder to an axial side of the hydraulic pump, the structure is complicated.
Furthermore, the present assignee has already proposed in Japanese patent publication Kokai No. 4963/87 a hydrostatic continuously variable transmission which is constructed to reliably deliver working oil between a hydraulic pump and a hydraulic motor and accordingly have a high transmission efficiency, wherein a pump cylinder of the hydraulic pump and a motor cylinder of the hydraulic motor are connected together to form a cylinder block, an annular inner oil passage and an annular outer oil passage surrounding the inner oil passage are formed between a number of annularly arranged cylinder bores of the pump cylinder and a number of annularly arranged cylinder bores of the motor cylinder, a number of first distributor valves and a number of second distributor valves are disposed in a radial arrangement, respectively, between the pump cylinder bores and the motor cylinder bores so as to reciprocally move between radially inner and outer positions for providing alternate connection of the number of pump cylinder bores with the inner and outer oil passages and alternate connection of the number of motor cylinder bores with those passages, respectively. Outer ends of the first distributor valves are engaged by a first eccentric ring and are thereby given reciprocating motion in response to relative rotation between the cylinder block and an input member of the hydraulic pump. Outer ends of the second distributor valves are similarly engaged by a second eccentric ring and are thereby given reciprocating motion in response to rotation of the cylinder block. The cylinder bores of the pump cylinder are each placed in communication with one of the oil passages in a discharge stroke and with the other oil passage in a suction stroke whereas the cylinder bores of the motor cylinder are each communicated with said one oil passage in an expansion stroke and with said other oil passage in a shrinkage stroke.
In the case of a hydrostatic continuously variable transmission as constructed above, however, the first and second eccentric rings for driving the first and second distributor valves are to be concentrated around the outer periphery of the cylinder block, as a result of which the dimension of the cylinder block becomes large in the radial direction. This may be an obstruction to obtaining a compact transmission.